This application relates generally to actuator servo control for disc drives and more particularly to normalizing the acceleration constant utilized by the servo control.
Disc drives store data on media and then read the data from the media when it is needed by the host computer. The data is contained in tracks on the media, and a read/write head is placed over the appropriate track by an actuator arm connected to a servo motor. An analog current signal is applied to the servo motor to move the actuator. The current applied to the servo must be precisely controlled to accurately guide the actuator and head to the appropriate track. The movement of the actuator and head from one track to another is known as a seek. It is generally desirable to reduce the seek time to a minimum.
A servo control system is included in the disc drive electronics to properly control the amount of current applied to the servo motor. A seeking profile is stored by the disc drive for each potential seek. The seeking profile is a function of the target velocity for the head for a given distance remaining to the desired track. The distance to the desired track at any given time in the seek is known as the position error. The seeking profile has an acceleration portion in which the actuator is accelerated from zero to its maximum velocity for the seek and a deceleration portion in which the actuator is decelerated from its maximum velocity for the seek back to zero. The seek profile places the read/write head over the desired track (i.e., zero position error) once the velocity has been reduced to zero.
The control system provides current to the servo motor by referencing the actual head position and the actual head velocity in relation to the seeking profile. The control system adjusts the amount of current applied by determining whether the actual head velocity is equal to the desired velocity stored in the seek profile for the current position error. To create a proper seek profile and determine the amount of current to be applied to the servo motor during the seek, the characteristics of the servo motor must be considered. An estimator, which is a model of the servo system, is included in the control system to predict the servo system""s response to commands intended to move the actuator as desired during the seek. The input command is converted to an analog current signal that drives the servo motor. The estimator has shortcomings because the gain variation of the servo motor may change over time due to aging and temperature and the estimator may become inaccurate due to that variation.
To cause the servo motor to behave like the estimator model and therefore, respond to the command as predicted, a correction factor is multiplied against the command used to drive the motor to cause the acceleration constant of the actual servo to match the acceleration constant of the estimator model. Alternatively, a correction factor may be multiplied by the estimator model to cause its response to match the actual servo. Several conventional methods of calculating the acceleration constant have been used.
However, the correction factor determined by these conventional methods causes the seek arrival to be asymmetric. A seek from one track to another in one direction results in an undershoot of the desired position while a seek between those tracks in the opposite direction results in an overshoot. Undershooting or overshooting the desired position requires additional time beyond that allotted in the seek profile for the head to settle over the desired position. This additional time is not desirable because it reduces the disc drive""s performance. Thus, the correction factor is not adequate, and arbitrary offsets must be added or control gains must be varied to attempt cancellation of the asymmetric seek behavior, thereby overly complicating the control system.
Accordingly there is a need for a method and apparatus that normalizes the acceleration constant of the servo motor or the estimator model so that each behaves similarly yet the servo motor does not behave asymmetrically without additional offsets and/or gain variances.
Against this backdrop the present invention has been developed. The present invention is a method and disc drive apparatus that normalizes the acceleration constant of the servo motor or estimator model by calculating a correction factor. The correction factor is computed by summing the position error of an actual seek from one track to another measured during the deceleration phase. That sum is then compared to the sum of position errors of a nominal (i.e., desired) seek for the same tracks measured during the deceleration phase.
The method disclosed is for normalizing an acceleration constant of an electromechanical control system of a disc drive by adjusting a correction factor implemented by the control system. The method involves measuring a test seek position error at each of several predetermined intervals during a deceleration phase of a test seek. The test seek position errors are arithmetically combined to obtain a test seek value. The correction factor is adjusted if the test seek value differs from a nominal seek value and otherwise is not generally adjusted.
The disc drive is configured to perform the method. The disc drive has at least one head having a controllable position. The head is configured to perform the measuring step of the method. The disc drive also has a processor configured to receive position errors from the head. The processor performs the combining step of the method and compares the test seek value to the nominal seek value. The processor also updates the correction factor in accordance with adjusting step of the method. The disc drive also has a servo control configured to implement the correction factor.